The present invention relates to a rotary drill, preferably of the type to be equipped with indexable cutting inserts. The drill is intended for forming a hole in a workpiece made of metal or the like. The inserts of the drill are radially disposed in the drill and are angled away from each other.
Rotary drills of this kind are previously known comprising two or more replaceable indexable inserts for boring, as exemplified by U.S. Shallenberger, Jr. Pat. No. 3,963,365 issued June 15, 1976. These prior drills, however, have had to be equipped with one or several wear strips since up to now it has not been thought possible to arrange the inserts such that radial forces exerted thereon could be balanced.
End mills equipped with pairs of inwardly and outwardly located inserts are exceptions, however. With such tools it is possible to have the cutting forces balanced as a result of the cutting edges being angled 180 degrees relative to one another, while such edges are directed radially outwards and perpendicularly to the feeding direction. Under these circumstances, very small radial side forces will be developed, if any. The small differences regarding cutting forces that might occur at the center of the drill compared with those at the periphery of the drill are simply compensated for by appropriate dimensioning of the length of the cutting edge of each insert.
With drills of the aforementioned type in which the cutting edge is located perpendicularly relative to the feeding direction of the drill (usually called the setting angle), appreciable disadvantages occur in conjunction with the outwardly located insert due to impaired chip removal, while the hole-generated cutting corner is subjected to unnecessary load. A disadvantage occurring with the centrally located insert is primarily an unnecessarily large zone with negative clearance angle which adversely affects the usable lifespan of the insert.
It is, therefore, an object of this invention to provide a drill with inserts so orientated relative to one another that the cutting forces acting on inserts located centrally and peripherally are essentially balanced.
It is another object of the invention to provide such a balanced drill wherein the need for wear strips is eliminated.
It is a further object of the invention to provide such a balanced drill wherein chip removal from the outwardly located insert is not impaired.
It is still another object of the present invention to provide such a balanced drill in which the cutting corner of the outermost insert is not subjected to unnecessary load.
It is yet another object of the present invention to provide such a balanced drill in which the inwardly located insert is not required to have an unduly large negative clearance angle.
It is still a further object of the present invention to provide a novel method for radially balancing a rotary drill.